1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling hybrid automatic repeat request (HARQ) feedbacks in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, uplink multiple-input multiple-output (UL-MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
The UE may receive packets (e.g., transport blocks (TBs)) transmitted by two eNBs, e.g., NB1-NB2, when the UE is configured with dual connectivity. The UE needs to transmit hybrid automatic repeat request (HARQ) feedbacks corresponding to the packets to the eNBs NB1-NB2, to acknowledge the reception of the packets. For example, the UE may transmit the HARQ feedbacks to the eNB NB1, and the HARQ feedbacks corresponding to the packets transmitted by the eNB NB2 may be forwarded by the eNB NB1 to the eNB NB2. However, a backhaul (e.g., X2 interface) connected between the eNBs NB1-NB2 may not be ideal, e.g., a delay caused by the backhaul to a transmission on the backhaul is large. In addition, resource for transmitting the HARQ feedbacks may depend on resource information which is only available at the eNB NB2. Thus, the eNB NB1 may not be able to transmit the HARQ feedbacks to the eNB NB2 via backhaul correctly, and the eNB NB2 may not operate regularly due to the delayed HARQ feedbacks. For example, the eNB NB2 may determine that the packets are not received correctly by the UE, and the eNB NB2 retransmits the packets to the UE. In this situation, not only throughput of the UE is degraded, but the UE cannot operate regularly due to wrong retransmissions of the packets.
Thus, it is important to mitigate or even eliminate the effect of the non-ideal backhaul between the eNBs, for the eNBs to receive the HARQ feedbacks immediately.